1. Field of the Invention
The invention is generally related to circular electron accelerators. More particularly, a combination of an internal electron injector and electrostatic deflection electrodes enhances the number of injected electrons that enter a main electron orbit of a Betatron and are accelerated to relativistic velocity.
2. Background of the Invention
Oil well bore hole logging is a process by which properties of earth strata as a function of depth in the bore hole are measured. A geologist reviewing the logging data can determine the depths at which oil containing formations are most likely located. Most present day well logging relies on gamma-rays obtained from chemical radiation sources to determine the bulk density of the formation surrounding a borehole. These sources pose a radiation hazard and require strict controls to prevent accidental exposure or intentional misuse. In addition, most sources have a long half life and disposal is a significant issue. For some logging applications, in particular determination of formation density, a 137Cs source or a 60Co source is used to irradiate the formation. The intensity and penetrating nature of the radiation allow a rapid, accurate, measurement of the formation density. In view of the problems with chemical radiation sources, it is important that chemical radiation sources be replaced by electronic radiation sources. The main advantage of the latter is that they can be switched off, when no measurement is made and that they have at most a very minimal potential for intentional misuse.
One proposed replacement for chemical gamma-ray sources is a Betatron accelerator. In this device, electrons are accelerated on a circular path by a varying magnetic field until being directed onto a target. The interaction of the electrons with the target leads to the emission of Bremsstrahlung and characteristic x-rays of the target material. Before electrons can be accelerated, they are injected into a magnetic field between two circular pole faces at the right time, with correct energy and correct angle. Control over timing, energy and injection angle enables maximizing the number of electrons accepted into a main electron orbit and accelerated.
In a typical Betatron, electrons are accelerated in a circular orbit by the EMF (electromotive force) induced by an increasing magnetic field. This requires that electrons be injected at the correct angle and energy at the right time. The injection angle is critical for optimal injection and needs to be controlled to better than one degree. Injection angle is typically controlled by proper alignment and positioning of an electron ejector. The injection angle can be fairly easily controlled in large Betatrons, i.e. with a circular magnetic field of 4.5 inches in diameter or larger, through the use of an external electron injector. One such external electron injector is disclosed in U.S. Pat. No. 6,713,976 to Zumoto, et al.
Large Betatrons are suitable for applications where size constraints are not critical, such as to generate x-rays for medical radiation purposes. However, in applications such as oil well bore holes where there are severe size constraints, it is desired to use smaller Betatrons, typically with a magnetic field diameter of 3 inches or less. With this size constraint, an external electron injector is not practical and a Betatron with an internal electron gun and injector is preferred. An internal injector is mounted inside the main vacuum chamber in close proximity to the electron orbit. One such Betatron is disclosed in U.S. Pat. No. 6,201,851 to Piestrup, et al. With an internal injector, accurate control of the injection angle becomes more difficult and adjustments after sealing the vacuum chamber are difficult or impossible. In addition to the cathode and an anode, an internal injector may include additional electrodes such as grids for improved electron extraction, pulsing and/or focusing or other electrodes required for improved electron optics. However, the direction of the beam exiting the injector is fixed and given by the geometry of the electron gun and the magnetic field.
U.S. Pat. Nos. 6,201,851 and 6,713,976 are incorporated by reference in their entireties herein.
There remains a need for an internal electron gun and injector having better control over the injection angle of electrons for use in a betatron having application for down hole well bore applications.